There are many well-developed applications and techniques known in the art for the replacement or augmentation of natural body parts with medical implants. These medical implants can be divided into two general classes of implanted medical devices. The first class includes implants which perform useful and essential functions based upon a variety of mechanical properties, including strength and flexibility. Examples of such implants include replacement heart valves and artificial joints. The second class includes implants which perform useful and essential functions by virtue of the physical shape of the implant rather than its structural or mechanical properties. Examples of this class of implants include cosmetic devices designed to augment or replace missing tissue or, more importantly, artificial optical lenses designed to augment or replace the natural lens of the eye.
Although medical implants of this second class have been successfully used for many years, their use is not without problems. One of the primary difficulties is the physical trauma caused by the surgical incisions that must be made in the body to position the implants. It is well known in the medical art that reducing the size of the surgical incision needed for the implantation procedure will reduce this trauma. At present, reducing the size of the surgical incision is best achieved, where possible, by reducing the size of the implant itself. Alternatively, recent research and development has focused on reducing the size of the surgical incision itself. Through the utilization of arthroscopic or microsurgery techniques and instruments, implanting surgeons can confine the physical impact of the surgical procedure to the desired target location through small, often remote incisions. These small incisions reduce much of the trauma normally associated with surgery using conventional large-incision techniques. As a result, much of the discomfort, healing time and complications that may occur can be reduced with small-incision techniques.
This research has not been easy because the volume, dimensions, and relative rigidity of the conventional implants place practical limits on the available reduction of incision size. Though relevant to many types of prosthetic and cosmetic implants, this problem is typified by artificial optical lenses, known as intraocular lenses or "IOLs". These artificial lenses, are implanted into the eye to replace or augment the natural lens and it ability to focus light onto the retina of the eye. In this functional context, it is the shape and volume of the lens, in conjunction with the refractive index of the lens material, that causes the light entering the eye and passing through the lens to be focused properly onto the retina permitting clear vision.
Presently, most practical intraocular lens implantation procedures require an incision in the eye that is greater than 3 millimeter (mm) to 4 mm. In most cases, an intraocular lens is implanted after the removal of the damaged or cataractous natural lens. Currently, the procedure for the removal of the natural lens requires an incision of at least 3 mm to 4 mm. However, the typical intraocular lens implant includes an optical light focusing lens portion and minor projecting structural features ("haptics") which assist with the placement and retention of the lens within the eye following implantation. Most currently available IOLs have a minimum diameter on the order of 6 mm and a minimum thickness of 1 mm to 2 mm. More recently, lenses known as "full-size optics", intended to completely replace the natural lens, have been developed having minimum diameters ranging from 8 mm to 13 mm and minimum thicknesses ranging from 3 mm to 5 mm. Thus, a surgical incision that is at least as large as the minimum dimension of the optical implant must be made. There are significant drawbacks to the use of any incision in the eye, especially ones that are greater than 3 mm to 4 mm. These drawbacks include post-operative astigmatism or corneal distortions, as well as the potential for increased complications and healing time.
One method known for reducing the size of the surgical incision associated with implanting an intraocular lens is to form the lens from a relatively flexible material which is folded or rolled to reduce the size of one dimension prior to inserting the lens into the eye. Once implanted, the lens is intended to unfold and return to its original shape. Foldable lenses, although adequate for their intended purposes, have drawbacks which limit their use for small-incision surgical implantation and may make them impractical. For example, when folded, only one of the three dimensions, the diameter or the width, can be reduced, and then, by only half. At the same time, one of the other dimensions, the thickness, is necessarily doubled while the third dimension remains unchanged. The minimum incision size is thus limited to one half of the largest dimension, which in the case of currently available lens configurations, remains on the order to 4 mm to 6 mm in length. Further compounding matters, folding the lens may produce permanent creases or deformation in the optical portion of the lens, causing visual distortion following implantation.
An alternative method that has been proposed for reducing incision size during implantation is the use of expansile lenses made of materials such as hydrogels. The hydrogel lens is desiccated prior to insertion to reduce the overall volume and dimensional characteristics of the lens. Following implantation, the hydrogel material is intended to rehydrate and expand back to its original size. While such hydrogel lenses are capable of significant reductions in size, the current state of the hydrogel art requires a re-hydration period following implantation ranging from 3 hours to 24 hours. This length of time is impractical because the implanting surgeon is unable to determine whether the lens is properly positioned in the eye prior to complete hydration. As a result, implanting surgeons may be reluctant to use such lenses because they require waiting prior to close the implantation incision until the surgeons are certain that access to the interior of the eye is no longer necessary to reposition the lens.
Other methods for the small-incision surgical implantation of intraocular lenses have been proposed with little success. In one proposal, a transparent balloon lens in its empty or deflated state is to be inserted into the eye through a small incision. Once inserted into the eye, the proposed balloon lens is to be filled with a highly refractive material to inflate the lens to its final configuration. To date, balloon lenses have proven to be impractical as they are difficult to manufacture and inflate with any degree of accuracy or control following implantation. Further, there are unsolved difficulties with materials, the removal of bubbles, and with the sealing of the lenses.
Similarly, injectable lenses have been proposed to replace the natural lens in situ wherein a liquid polymer would be injected into the naturally occurring lens capsule and allowed to cure into its final configuration. Present technology has been unable to produce such lenses because it is difficult to produce predictable optical power and resolution with biocompatible materials.
A more practical and realizable method for reducing the size of the surgical incision used when implanting an intraocular lens is disclosed in currently pending U.S. patent application Ser. No. 08/607,417, Now U.S. Pat. No. 5,702,441. With this technique, lenses are formed from a memory material, i.e., a material having the ability to be shape transformable, such as elastomeric or gelatinous materials capable of substantial recoverable deformation in all directions. These lenses are implanted through a small incision in the eye using a small-diameter, tubular ejector. Following implantation, the gelatinous lens implants immediately reassume their pre-implant shapes and configurations, allowing the implanting surgeon to confirm proper placement and completion of the implantation procedure.
However, even this technology can be improved upon. For example, when such lenses are deformed and placed within the tubular ejector, the lenses are forced into a shape having a high surface-area-to-volume ratio. Under these conditions, there may be strong elastomeric forces exerted by the deformed lens on the tubular ejector as the deformed lens tries to recover its original size and shape. These forces, coupled with the large surface-area-to-volume ratio, may cause the deformed lens to be difficult to push out of the tubular ejector and into the eye.
Accordingly, one of the objectives of the present invention is to provide implantation methodology that will allow the rapid and easy insertion and positioning of medical implants through very small surgical incisions relative to the size of the implant without the use of complicated or sophisticated techniques or implant delivery systems.
It is an additional object of the present invention to provide surgical implants such as intraocular lenses that can be inserted and positioned within a patient through a very small incision relative to the shape, size, and volume of the implant.
It is yet another object of the present invention to provide stretch-crystallizable silicone intraocular lenses that are optically clear, have high refractive indices, and that can be stretched into long, thin rods or blades which crystallize and stabilize at temperatures below normal body temperature, and which reassume their pre-stretch-crystallized shape, contours and physical characteristics within seconds after being implanted into the eye.